A navigation apparatus provides course guidance for travel to a destination to a driver who is unfamiliar with the local geography. Recent years have seen extensive development of such navigation apparatus.
The conventional navigation apparatus is adapted to set a course from a point of departure to a desired destination by being provided with an input of point of departure and destination prior to travel, and to perform navigation in accordance with the course set. In a case where a course is designated, navigation is such that a map is displayed on the screen of a CRT and the course is superimposed on the map. In some apparatus, distance to an intersection at which a turn is to be made next is displayed numerically, in the form of a graph or as a characteristic photograph as information relating to the intersection at which the next turn is to be made in accordance with the preset course. A voice-track output is also used in some apparatus.
However, with the conventional navigation apparatus, a course is set from a departure point to a destination and course guidance is provided to the driver in accordance with the course set, as mentioned above. Consequently, a problem is that if the driver should happen to mistake, say, an intersection and stray from the course, travel in accordance with the guidance provided by the navigation apparatus will not be able to continue unless the vehicle is returned to the set course or a course is set anew with the prevailing position serving as the point of departure. However, both returning to the set course and setting a course with the prevailing position as the departure point place a great load upon the driver. In other words, the fact that course guidance is required means that the driver is not familiar with the road conditions in the region. If the driver loses his or her way in such an unfamiliar region, it will be difficult to ascertain what the present position is, even though the departure point and destination may be known.
Furthermore, a decision as to whether or not a predetermined intersection has been passed as specified by the course guidance is premised upon detection of travelled distance or a left or right turn as detected by a distance sensor or steering angle sensor, respectively. In actuality, however, detection errors occur and can accumulate and cause errors in judgment. With the conventional navigation apparatus, course from departure point to destination is set and travelling distance is added up while left and right turns are detected in accordance with the course. Consequently, distance errors are also added up until a correction can no longer be applied. Furthermore, only present position on a course can be specified. If an angle on a branch road having only small angular differences lies within an allowable error, a decision to the effect that the vehicle is travelling correctly will be made based solely on this meager information. As a result, judgment errors are likely to occur.
An object of the present invention is to enable present position to be calculated reliably at every intersection at any geographical point without a course being fixed. Another object of the present invention is to enable appropriate guidance information to be outputted at any geographical point. A further object of the present invention is to enable present position to be recognized at an intersection and to correct distance errors.